The present invention generally relates to gas separation membranes and, more particularly, to aromatic polyimide gas separation membranes prepared from contacting acid dianhydrides with diamines.
Gas separation membranes (GSM) may be employed to separate a particular gas from a mixture of gases. For example, a GSM may be used to separate and remove oxygen from air so that a resultant gas mixture will not be combustible. GSM's may also be employed to separate carbon dioxide (CO2) from methane (CH4) and hydrogen (H2) from CH4.
Many GSM's are formed from polymers with molecular structures that have intermolecular spacings sufficiently small that they can discriminate between molecules of differing size, such as CH4 and H2, but with high enough fractional free volume that these gases can diffuse through the GSM. The degree to which the GSM provides selective blocking of some gases and passages of others is referred to as “selectivity” of the GSM.
Useful membranes for separating gases must have sufficient selectivity to distinguish between two gases, and must also have high flux. Flux is generally quantified as either permeance or permeability. Permeance, measured in Gas Permeation Units (GPU), is the gas flow (measured as cubic centimeter, cm3, at standard temperature and pressure (STP)), per membrane area (cm2), per trans-membrane pressure drop (cm Hg), per unit time (second or s): 1 GPU=10−6 cm3 (STP)/cm2 s (cm Hg). Permeability, measured in Barrer, is the permeance multiplied by the skin layer thickness of the membrane 1 Barrer=10−10 cm3 (STP) cm/cm2 s (cm Hg). Flow through the membrane will increase with increasing membrane area or trans-membrane pressure drop, and will decrease with increased membrane selective layer thickness.
In this context, it is desirable that a GSM have low selective layer thickness. Indeed the lower the thickness of the selective layer, the higher the flux. But, thin GSM's tend to be fragile. Consequently, in many practical applications, a GSM may be supported on a base support structure. Such a support structure may be substantially porous with respect to the gas mixtures that are the subject of separation. In other words, a material for a support structure may be selected to provide strength while not reducing flux.
As can be seen, there is a need to provide a GSM that may provide high separation selectivity for a gas mixture and a high flux. There is also a need to provide such a GSM in a structure that has sufficient strength for use in practical applications.